1. Field of the Invention
This invention relates to microwave electronic devices employing yttrium iron garnet, and, in particular, to a process for providing lanthanum-doped yttrium iron garnet disks suitable in such devices.
2. Description of the Prior Art
Yttrium iron garnet (Y.sub.3 Fe.sub.5 O.sub.12 ; YIG) is an important material for microwave elecronic devices because of its high Q value at microwave frequencies. Currently, spheres of YIG are widely used as narrow band filters, microwave resonators and the like. The spheres are individually fabricated from flux grown YIG single crystals by a a crystal growth process which commonly requires a few weeks. The fabrication of crystals into small spheres is time consuming and requires sophisticated polishing techniques.
Liquid phase epitaxy (LPE) growth of YIG films on gadolinium gallium garnet (Gd.sub.3 Ga.sub.5 O.sub.12 ; GGG) subtrates, with subsequent processing into photoetched disks, has been disclosed in Vol. MAG-9, IEEE Transactions on Magnetics, pp. 535-7 (1973). Since YIG has a smaller lattice parameter than GGG, Pb.sup.2+ ions were incorporated in the YIG lattice to achieve a closer match of lattice parameter constants in order to reduce strains that would otherwise arise due to lattice mismatch. The lead content, however, apparently resulted in a broadening of the ferromagnetic resonance linewidth, thus rendering the YIG disks less suitable for microwave resonant applications. Further, during processing of the YIG film to photoetch YIG disks, sputtered SiO.sub.2 was used to mask the YIG film. The sputtered SiO.sub.2 apparently resulted in surface damage of the YIG film, in consequence of which ferromagnetic resonance linewidth was further broadened.
A study of dependence of lattice parameter on composition in substituted yttrium iron garnet epitaxial layers grown on GGG substrates has been published; see, e.g., Vols. 17 and 26, Journal of Crystal Growth, pp. 322-328 (1972) and 122-126 (1974), respectively. In the study, yttrium is substituted by gadolinium, samarium and lanthanum, while iron is substituted by gallium. No useful methods for fabricating microwave devices are given, however.